Mathematical functions for mesh in Blender 2.8 beta
The addon is still there - you can find it in \scripts\addons\add_mesh_extra_objects within the Blender folder. However, it hasn't yet been updated to be compatible with Blender 2.8 and so is not shown in the list.

Within the __init__.py file within each addon is a section that defines the 'bl_info' that tells Blender about the addon :

bl_info = {
    "name": "Extra Objects",
    "author": "Multiple Authors",
    "version": (0, 3, 2),
    "blender": (2, 74, 5),
    "location": "View3D &gt; Add &gt; Mesh",
    "description": "Add extra mesh object types",
    "warning": "",
    "wiki_url": "https://wiki.blender.org/index.php/Extensions:2.6/"
                "Py/Scripts/Add_Mesh/Add_Extra",
    "category": "Add Mesh",
}


The version section tells Blender what versions this addon is compatible with - in this case 2.74.5. Normally an addon would be deemed to be compatible with anything greater than this version number but it appears that Blender 2.80 includes some changes that mean that only addons that are specifically targeted at 2.80 and later will be considered.

You can try to get an addon to be accepted by Blender by amending its __init__.py to change the version to (2,80,0) - and this will get it to be listed in the list of available addons - but it is likely to fail when you try and activate it.

For example, it appears that the bpy.utils.register_module(...) function has now been removed in preference to byp.utils.register_class(...) - so any addon that uses the older method will now need to be amended to use the new methods. This will require each addon writer to update his/her addon to make it compatible.